[DOCS] showcases, examples & general docs improvements

:mega: THIS IS A CALL FOR DOCS
Feel free to add pull requests with examples here:
- https://github.com/CycloneDX/cyclonedx-python-lib/tree/main/examples
- https://github.com/CycloneDX/cyclonedx-python-lib/blob/main/docs/examples.rst

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There are currently no (complex) examples how to use this lib.

Yes, there are API docs, and small snippets, but no examples.
Even the unit tests do now showcase example usage, as they are much declarative or not expressive enough.

GOAL:
- [x] have examples in a `examples` folder
- [x] link the examples in the README
- [x] link the examples in the (api)-docs
- [x] prevent `examples` dir from being shipped (sdist/bdist)
- [x] have examples run in CI

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